Endothelin-1 Stimulates Aldosterone Synthesis in Conn's Adenomas via Both A and B Receptors Coupled with the Protein Kinase C- and Cyclooxygenase-Dependent Signaling Pathways

Abstract

Background

The mechanisms and factors leading to enhanced aldosterone secretion and ultimately to neoplastic transformation of the adrenal cortex are poorly defined. Angiotensin-II (Ang-II) and endothelin-1 (ET-1) have emerged as likely candidates among potential aldosterone secretagogues and adrenocortical growth-promoting factors. We therefore compared the effects of Ang-II and ET-1 on steroid hormone secretion of Conn's adenomas.

Methods

Ten Conn's adenomas that showed responsiveness to Ang-II blockade in vivo were recruited. Fragments of the tumors were collected immediately after surgical excision, and dispersed cells were obtained by collagenase digestion and mechanical disaggregation. Steroid hormones secreted by dispersed Conn's adenoma cells were assayed by quantitative high-performance liquid chromatography or radioimmunoassay.

Results

Both Ang-II and ET-1 (10⁻⁹ mol/L) similarly enhanced the overall steroid hormone production. ET-1 raised the release of pregnenolone (as evaluated by blocking its further metabolism by cyanoketone), corticosterone, 18-hydroxycorticosterone, and aldosterone, without affecting that of 11-deoxy-cortisol, cortisol, and 11-deoxycorticosterone. The hormonal responses to ET-1 were partially reversed by 10⁻⁷ mol/L of either the ETA-receptor antagonist BQ-123 or the ETB-receptor antagonist BQ-788 and were abolished when both antagonists were used together. The aldosterone response to the selective activation of ETA and ETB receptors was studied in three Conn's adenomas by exposing dispersed cells to ET-1 (10⁻⁹ mol/L) plus BQ-788 (10⁻⁷ mol/L) and to the ETB-receptor agonist BQ-3020 (10⁻⁸ mol/L). Both treatments raised aldosterone output by about 2-fold. ETA receptor-mediated aldosterone response was abolished by the protein kinase (PK) С inhibitor calphostin C (10⁻⁵ mol/L). ETB receptor-mediated secretory response was lowered by either calphostin C and the cyclooxygenase (COX) inhibitor indomethacin (10⁻⁵ or 10⁻⁴ mol/L) and was completely suppressed when these two were combined. The PKA inhibitor H-89 and the lipoxygenase inhibitor phenidone were ineffective.

Conclusions

Collectively, our findings indicate that Ang-II and ET-1 equipotently stimulate both early and late steps of aldosterone synthesis in Conn's adenoma cells. The secretagogue effect of ET-1 occurs via the activation of ETA and EТв receptors, which are coupled with the PKC-dependent and the PKC- and COX-dependent signaling pathways, respectively.